SUMMARY Like most genetic disorders, no specific therapeutic intervention targets the molecular defect of Prader-Willi syndrome (PWS), a genomic imprinting and neurobehavioral disorder that significantly affects the quality of life of affected individuals. PWS is caused by paternal deficiency of genes in the chromosome 15q11-q13 region. The corresponding genes on the maternal chromosome are structurally intact, but their transcription is repressed epigenetically. The involvement of epigenetic regulation renders PWS one of the best opportunities to explore molecular therapy. Recent reports indicate that SNORD116, a SnoRNA cluster located between the SNRPN and UBE3A genes, is responsible for key features of PWS. Although DNA methylation and chromatin modifications at the PWS imprinting center (PWS-IC) are believed to regulate the silent expression of PWS genes in the maternal 15q11-q13 region, the exact mechanism remains elusive. Thus, one attractive molecular-based, therapeutic strategy for PWS is to unsilence the expression of paternally expressed PWS genes, primarily SNORD116, from the maternal chromosome. Because SNORD116 is processed from the long noncoding host RNAs initiated from the PWS-IC or Snrpn promoter, we developed a drug screening system using mouse embryonic fibroblasts (MEFs) derived from mice carrying a maternal Snrpn-EGFP fusion protein. In collaboration with Dr. Bryan Roth (consultant for this proposal), Dr. Jiang (PI) screened 9200 small molecules and identified and validated two compounds that can unsilence the expression of both Snrpn and Snord116 in human PWS cells and a PWS mouse model. These compounds are selective inhibitors of histone methyltransferases (HMTs), as defined by Dr. Jin (co-PI), whose research group is a leader in discovering selective inhibitors of HMTs. Interestingly, in contrast with reactivation of SNRPN by DNA methylation inhibitors, these compounds reduced the H3K9 methylation level but did not change DNA methylation of the PWS-IC. These observations together offer new insights and opportunities to investigate the mechanism underlying the imprinted expression of PWS genes. Our central hypothesis is that these compounds unsilence PWS candidate genes by modifying epigenetic complexes in the PWS-IC, which will provide clinical benefits in PWS mouse models. We propose a Chromatin Spreading Model mediated by H3K9 methylation as a mechanism of imprinted regulation of PWS genes. Our long-term goal is to launch a clinical trial using these compounds or their derivatives in human PWS. The complementary expertise and close collaboration between Dr. Jiang (molecular and human genetics of PWS) and Dr. Jin (chemical biology of novel epigenetic drug development) uniquely position them to attain the specific objectives of this study, which are to understand the mechanism by which these compounds unsilence PWS candidate imprinted genes, to evaluate their efficacy and toxicity, and to optimize their drug-like properties. The proposed study is significant because it will provide novel insight into the molecular mechanism underlying genomic imprinting in PWS and lead to the development of a therapeutic intervention for the disease.